USB-C is the physical connector. Thunderbolt is a high-speed protocol that runs over USB-C. Thunderbolt 4/5 supports 40/80 Gbps; USB 3.2 supports 5/10/20 Gbps; USB4 supports up to 40 Gbps.
Connector vs protocol confusion: every Thunderbolt port is USB-C, but not every USB-C port is Thunderbolt.
USB-C protocols: USB 2.0 (480 Mbps, charging-only), USB 3.2 Gen 1 (5 Gbps), Gen 2 (10 Gbps), Gen 2×2 (20 Gbps), USB4 (20–40 Gbps).
Thunderbolt: TB3 / TB4 (40 Gbps), TB5 (80 Gbps with Bandwidth Boost up to 120 Gbps for displays).
Thunderbolt requires Intel/Apple silicon controller. Mainly found on premium laptops (MacBook Pro, Dell XPS, ThinkPad). Use cases: external GPUs, dual 4K monitor support, fast SSD enclosures.